I've installed SDDM from Sid. I get login screen, enter username/password and hit Enter and ... get the SDDM login screen again. Endless loop.
Using Ctrl Alt F4 I get shell login, and do so. In the shell I enter "startx" and the KDE tandwheel appears and I get the KDE desktop. Nice!
How to debug SDDM? Where to look for debug info.